1. Group column, shows groups' name which students are enroled. In the "All participants" export (without selection of group), you'll see a row for each student-group couple, it means that for the same student could have as many rows as groups which he was affiliated.

2. Date columns, for each grade item, shows the last modification date for this item grade (Y.m.d format)

Comments

It would be good to share this properly so here is some feedback in how we can go about that :
- It is better to include some urls here in the 'useful links' section:
-- public code repository (eg: github)
-- issue tracker (e.g: tracker.moodle.org or github.com//issues)
-- if you have any additional documentation you can create a link for it as well.

Generally, this feels like it could become a generalised feature and become included in core so you might want to consider improving XLS report with a patch in an MDL.